Polished Crystals Research Articles

Experimental modelling of corona textures

AbstractFormation of corona textures along olivine–plagioclase and orthopyroxene–plagioclase interfaces has been experimentally reproduced at 670 and 700 °C and 5 kbar with either a pure H2O fluid phase or 0.1 and 37 m NaCl–H2O solution fluid. In these experiments, we investigate the interaction of primary olivine and/or orthopyroxene and plagioclase in powders and polished crystals, and in small samples of a natural gabbro. The experiments result in the formation of corona textures with several layers of different assemblages (according to the experimental conditions) consisting of garnet (grossular), clinopyroxene, orthopyroxene, amphibole, chlorite and phlogopite. The experiments show major differences in the number of layers, the mineral assemblages and mineral composition, and in the trends of composition of plagioclase in coronas around olivine and orthopyroxene. The fluid phase composition influences the corona assemblages and the composition of the minerals in the experimental coronas; for example, garnet appears in the coronas in the second experiment where the NaCl–H2O ratio is low. Experimental modelling of corona textures confirms a model of simultaneous growth of layers by the mechanism of diffusion metasomatism with participation of a fluid phase through which mass is transferred. Zoning in the experimental coronas shows opposing diffusion of Al and Ca from plagioclase and Mg and Fe from olivine/orthopyroxene; difference in the mobility of the components is inferred from observations in the coronas. The experimental corona textures are compared with natural coronas from the Belomorian belt (Baltic shield), developed at 670–690 °C and 7–8 kbar, and the Marun‐Keu complex (Polar Urals), developed at 670–700 °C and 14–16 kbar, where the corona textures correspond to a transitional stage of the gabbro‐to‐eclogite transformation.

MESFETs on H-terminated Single Crystal Diamond

AbstractEpitaxial diamond films were deposited on polished single crystal Ib type HPHT diamond plates of (100) orientation by microwave CVD. The epilayers were used for the fabrication of surface channel MESFET structures having sub-micrometer gate length in the range 200-800 nm. Realized devices show maximum drain current and trasconductance values of about 190 mA/mm and 80 mS/mm, respectively, for MESFETs having 200 nm gate length. RF performance evaluation gave cut off frequency of about 14 GHz and maximum oscillation frequency of more than 26 GHz for the same device geometry.

Deformation of soft crystals in precision polishing

Grinding/polishing soft materials typically introduces a layer with residual stresses, which impedes the examination of the original deformation states. In this work, precision polishing procedures consisting of mechanical polishing and electrochemical polishing were used to obtain residual-stress-free surfaces in an annealed copper polycrystal and a copper single crystal. For the polycrystal, mechanical polishing followed by electrochemical polishing were used to show the grain structure and etching pits. The polished single crystal was analysed by electron backscatter diffraction (EBSD) technique to reveal the lattice rotation field related to the original deformation state induced by wedge indentation. Specifically, the geometrically-necessary dislocation density was calculated based on the in-plane crystal lattice rotation angles. The precision polishing makes the observation of the effective slip systems as predicted by the crystal plasticity theory possible. The precision polishing also helps reveal the deformation instability in the indented copper single crystal by sharp indenters.

A new experimental thin film approach to study mobility and partitioning of elements in grain boundaries: Fe-Mg exchange between olivines mediated by transport through an inert grain boundary

A new experimental method is presented where transport and partitioning of elements in grain boundaries is investigated by using thin films. As a model system, the element exchange between a nanocrystalline thin film of olivine (Fo 30 ) and a San Carlos olivine single crystal (Fo 89 ), which is mediated by grain diffusion through a polycrystalline ZrO 2 layer was studied. Double layers of ZrO 2 and olivine on polished single crystals have been prepared by pulsed laser deposition and annealed in the temperature range between 900 and 1050 °C at controlled oxygen fugacity. Starting and annealed samples were analyzed by SEM, RBS, and SIMS. From the chemical analysis by RBS and SIMS, the reaction mechanism could be classified according to the reaction mechanism map of Dohmen and Chakraborty (2003). Most of the samples show a combined grain boundary/volume diffusion-controlled reaction behavior according to this scheme. During the anneal, a linear concentration gradient for the Fe/(Fe + Mg) ratio evolved within the ZrO 2 layer, where Fe strongly prefers the ZrO 2 grain boundary compared to Mg. The depth profiles for the Fe/(Fe + Mg) ratio have been simulated in detail. From the simulation Fe-Mg volume (in olivine) and grain boundary diffusion coefficients, as well as the partition coefficient and the storage capacity for the grain boundary, have been extracted. The chemical diffusion coefficient of Fe-Mg within the ZrO 2 grain boundary at 900 °C is on the order of 1 × 10 −15 m 2 /s. Volume and grain boundary diffusion coefficients obtained using the new method are in good agreement with literature data. An additional observation is that grains within the polycrystalline fayalite-rich olivine layer grow by a grain rotation mechanism until an equilibrium texture is attained.

Influence of substrates on roughness of self-supporting Ni films

Self-supporting Ni foils were obtained by floating of Ni films from water soluble substrates. Ni films were prepared by metal vapor vacuum arc (MEVVA) ion deposition. The following release agents were applied on Si wafer substrates: betaine monohydrate with sucrose, potassium oleate, potassium oleate with sucrose. In addition polished NaCl crystals and self-supporting collodion foils were used as soluble substrates, respectively. Field emission scanning electron microscopy (FESEM) and atomic force microscope (AFM) were employed to analyze the surface morphology and the roughness of the Ni films and the substrates. The results indicate that the mean roughness of the self-supporting Ni foils depends on their substrates. Self-supporting collodion foils seem to be one of the most suitable candidates for preparing self-supporting Ni foils with low roughness. Mean roughness of the best self-supporting Ni foils is about 3.8 nm.

Scattering of light molecules from Al2O3 surfaces

Various light ions were scattered at grazing incidence from a highly polished Al2O3 single crystal and from a thin film of Al2O3 on graphite. The energy of incident particles was varied from 390eV to 1200eV. For scattered positive oxygen and carbon ions, negative ion fractions of up to 14% and 3%, respectively, were recorded. For scattered positive hydrogen ions, the negative ion fractions reached the percent level.

Oxygen diffusion in titanite: Lattice diffusion and fast-path diffusion in single crystals

Oxygen diffusion in natural and synthetic single-crystal titanite was characterized under both dry and water-present conditions. For the dry experiments, pre-polished titanite samples were packed in 18O-enriched quartz powder inside Ag–Pd capsules, along with a fayalite–magnetite–quartz (FMQ) buffer assemblage maintained physically separate by Ag–Pd strips. The sealed Ag–Pd capsules were themselves sealed inside evacuated silica glass tubes and run at 700–1050 °C and atmospheric pressure for durations ranging from 1 h to several weeks. The hydrothermal experiments were conducted by encapsulating polished titanite crystals with 18O enriched water and running at 700–900 °C and 10–160 MPa in standard cold-seal pressure vessels for durations of 1 day to several weeks. Diffusive uptake profiles of 18O were measured in all cases by nuclear reaction analysis (NRA) using the 18O (p,α) 15N reaction. For the experiments on natural crystals, under both dry and hydrothermal conditions, two mechanisms could be recognized as responsible for oxygen diffusion. The diffusion profiles showed two segments: a steep one close to the initial surface attributed to self diffusion in the titanite lattice; and a “tail” reaching deep into the sample attributable to diffusion in a “fast path” such as planar defects or pipes. For the experiments on synthetic crystals, lattice diffusion only is apparent in crystals with euhedral morphology, while both mechanisms operate in crystals lacking euhedral morphology. For the dry experiments, the following Arrhenius relation was obtained: D dry lattice = 3.03 × 10 − 8 exp (− 276 ± 16 kJmol − 1 /RT) m 2/s Under wet conditions at P H 2O = 100 MPa, Oxygen diffusion conforms to the following Arrehenius relation: D wet lattice = 2.05 × 10 − 12 exp (− 180 ± 39 kJmol − 1 /RT) m 2/s Diffusive anisotropy was explored only at hydrothermal conditions, with little evidence of diffusive anisotropy observed. Oxygen diffusivity shows no dependence on water pressure 800 °C ( P H 2O = 10–160 MPa) or 880 °C ( P H 2O = 10–100 MPa). However, like many other silicates, titanite shows a lower activation energy for oxygen diffusion in the presence of H 2O than under dry conditions; therefore the difference between the “dry” and “wet” diffusivities increases as temperature decreases below the range of this study. For example, at 500 °C, dry diffusion is almost 2.5 orders of magnitude slower than wet diffusion. Accordingly, the retentivity of oxygen isotope signatures will be quite different between dry and wet systems at geologically interesting conditions. For most cases, wet diffusion results may be the appropriate choice for modeling natural systems.

Optical characteristics of GaN single crystals grown by the HVPE: Effects of thermal annealing and N 2 plasma treatment

Single-crystalline 350-μm-thick GaN were grown on 2 in (0 0 0 1) sapphire substrate by the hydride vapor phase epitaxy method. The free-standing bulk-like GaN crystals were obtained by delaminating from the sapphire substrate by the laser-induced lift-off technique. Mechanical polishing of the GaN crystals to remove the damage layer on their surfaces resulted in deteriorating optical properties. In order to recover the optical damages induced by the mechanical polishing, thermal annealing and N 2 plasma treatment were adopted. As the annealing temperature was increased up to 900 °C, the surface morphology and photoluminescence property of the polished GaN crystals improved, resulting from removal of the polishing-induced damage layer on the surface. Moreover, the N 2 plasma treatment on the mechanically polished samples effectively removed the polishing-induced damage, leading to remarkably improve the surface morphology and photoluminescence.

Optimization of the effective light attenuation length of YAP:Ce and LYSO:Ce crystals for a novel geometrical PET concept

The effective light attenuation length in thin bars of polished YAP:Ce and LYSO:Ce scintillators with lengths of the order of 10 cm has been studied for various wrappings and coatings of the crystal lateral surfaces. This physical parameter plays a key role in a novel 3D PET concept based on axial arrays of long scintillator bars read out at both ends by Hybrid Photodetectors (HPDs) since it influences the spatial, energy and time resolutions of such a device. In this paper we show that the effective light attenuation length of polished crystals can be reduced by wrapping their lateral surfaces with Teflon, or tuned to the desired value by depositing a coating of Cr or Au of well-defined thickness. The studies have been carried out with YAP and LYSO long scintillator bars, read out by standard photomultiplier tubes. Even if the novel PET device will use different scintillators and HPD readout, the results described here prove the feasibility of an important aspect of the concept and provide hints on the potential capabilities of the device.

Micro-Raman analysis on LiNbO 3 substrates and surfaces: Compositional homogeneity and effects of etching and polishing processes on structural properties

In this work we report on micro-Raman analysis on lithium niobate (LN) substrates in order to study the compositional homogeneity of the crystals and to clear up the effects of etching and polishing processes on the surface of wafers and crystals. The fact that the linewidth of some Raman modes scale with the composition of LN crystals, together with the use of a confocal microscope, allowed a three-dimensional determination of the sample stoichiometry and of the crystalline quality. This local tool can supply additional information, which can be complementary to the electro-optic coefficients, carefully measured as well in order to check functional parameters. Raman spectra from buried regions were obtained on as-grown, etched and polished crystals and wafers. The depth profile of the peak energy and the linewidth of the Raman mode at 872 cm −1 indicate that mechanical processing of surfaces causes, in some cases, structural modifications till a depth of 15 μm.

Growth and characterization of undoped and thallium doped cesium iodide single crystals

Single crystals of pure and thallium (Tl) doped cesium iodide (CsI) have been grown by melt growth (Bridgman) technique. The grown crystals were subjected to powder X-ray diffraction and high-resolution XRD analysis. The cut and polished crystals were characterized for luminescence studies. UV-visible transmission studies have been carried out on the grown crystal in the wavelength range 200–650 nm. From the transmission spectrum it was found that the cut off wavelength increases with increase in Tl concentration and the transmittance is about 70%. The 0.06 mol% of Tl doped CsI crystal shows a good energy resolution of 7.6%. The hardness decreases for increasing the doping concentration. Etching studies have been carried out on doped and undoped crystals using methanol and water as etchant.

Synthetic [100] tilt grain boundaries in forsterite: 9.9 to 21.5°

Pure forsterite bicrystals were produced by direct bonding of highly polished single crystal plates. We fabricated a series of synthetic grain boundaries with tilt axis a and tilt angles ranging from 9.9 to 21.5°. Transmission Electron Microscope (TEM) investigations show low-angle grain boundaries with arrays of c-dislocations for all misorientations with tilt angles up to 21.5°. Low-angle grain boundary structures were fully developed at annealing temperatures <400°C and no structural changes were observed in the grain boundary region after further annealing at 1650°C.

Influence of the crystal-surface unevenness on the angular spread of an x-ray diffracted beam

One of the factors influencing the focus size in diffractive–refractive optics is the quality of diffracting surface. If the surface is uneven, as it is when the silicon crystal surface is only etched, then the diffraction at each point of the surface is a combination of an asymmetric and inclined diffraction (general asymmetric diffraction). This somewhat deviates and spreads the diffracted beam. The integration over the surface hit by an incident beam gives the angular spread of the diffracted beam. It is shown theoretically that in some cases (highly asymmetric, highly inclined cut) the etched surface may create the spread of the diffracted beam such that it causes a significant broadening of the focus. In this case a mechanical–chemical polishing is necessary. This has been verified by us earlier in a preliminary experiment with synchrotron radiation. In this work the new experiment with the same crystals is performed using double crystal (+, −) arrangement and a laboratory x-ray source (CuKα radiation). We compared two samples; one of them is mechanically–chemically (MC) polished and thus the diffracting surface is almost perfect; the other is only etched. This experiment allows a better comparison of the result with the theory. The difference between the measured rocking curve widths for the etched and MC polished crystals (10″) roughly agrees with theory (7″), which supports the correctness of the theoretical approach.

Formation of Optical Waveguide and Annealing Behaviour ofLiNbO3 Implanted by 3.0-MeV Nickel Ions

The optically polished LiNbO3 crystal was implanted with 3.0 MeVnickel ions to a dose of 8×1014 ions/cm2. Theimplanted sample was orderly annealed at different temperatures fordifferent times in air ambient. The waveguide properties before andafter annealing were investigated. Dark modes were observed by theprism coupling technique. The reconstructed refractive index profileswere obtained with the reflectivity calculation method. With the fibreprobe technique, the attenuation of the Ni-implanted barrier-confinedwaveguide was measured for the first time. The waveguide loss wasreduced from 1.65 dB/cm to 1.19 dB/cm when annealing was at 300 degrees Cfor 30 min. With the annealing temperature increasing to 370 degrees C,only fewer broad dark modes were found. It is indicated that thelattice damage by implantation began to resume under this annealingcondition. After annealing at 380 degrees C for 60 min, all the darkmodes disappeared.

Microstructure of 24-1928 Ma concordant monazites; implications for geochronology and nuclear waste deposits

The microstructure of monazite was studied using scanning electron microscopy (SEM), electron microprobe analysis (EMP), X-ray diffraction patterns (XRD), and transmission electron microscopy (TEM). Four well-characterized monazites were investigated, having very different concordant U-Pb ages (24 to 1928 Ma), and up to ∼15 wt.% ThO 2, and ∼0.94 wt. % UO 2. The SEM and EMP analyses of polished single crystal fragments reveal the absence of significant chemical zoning. XRD and TEM investigations show that the monazites are not metamict, despite their old ages, very high abundances of radionuclides, and hence, high time-integrated radiation doses. Except for the youngest one, the monazite crystals are composed of a mosaic of crystalline but slightly distorted domains. This structure is responsible for the presence of (1) mottled diffraction contrasts on the TEM, and (2) a second structural phase (B), with very broad reflections in the XRD patterns. Older monazites receive higher self-irradiation doses, and hence, they contain higher amounts of this B-phase. For the 1928 Ma monazite, XRD reveals only the broad reflections of phase B, implying that the whole monazite was affected by radiation damage that resulted in total distortion of the lattice. It is concluded that radiation damage in the form of amorphous domains does not accumulate in monazite because self-annealing heals the defects as they are produced by α-decay damage. The only memory of irradiation-induced defects is the presence of distorted domains. As the diffusion rate of Pb in an undisturbed monazite lattice is extremely low, Pb loss due to volume diffusion out of the monazite lattice is virtually impossible. This is considered as one reason why almost all monazites have concordant U-Th-Pb ages. Moreover, as long-term self-irradiation effects are limited in monazite, we consider this phase as a good candidate for the storage of high-level nuclear waste under the aspect of its high resistance to irradiation.

Application of the Quartz Crystal Microbalance to the Evaporation of Colloidal Suspension Droplets

An investigation into the evaporation of sessile droplets of latex and clay particle suspensions is presented in this work. The quartz crystal microbalance (QCM) has been used to study the interfacial phenomena during the drying process of these droplets. Characteristic changes of the crystal oscillating frequency and crystal resistance (damping of the oscillating energy) have been observed and related to the different stages of the evaporation process. Measurements have been made for latex particle sizes from 1.9 to 10 microm and for rough and polished crystals using drops from 0.3 to 1.5 microL. The behavior of the QCM is shown to depend strongly on the size of particles present and on the morphology of the crystal surface. One of the most striking features is a drastic damping of the oscillation energy and corresponding rise in frequency observed during the final stages of evaporation, particularly for the clay suspensions.

Spontaneous recovery of hydrogen-degraded TiO2 ceramic capacitors

Degradation of TiO2-based ceramic capacitors was observed after hydrogen incorporation on the termination electrodes of the capacitors via electrolysis of water. Fourier-transform infrared (FTIR) absorption spectra analysis of polished rutile single crystals clearly showed that hydrogen was incorporated into the TiO2 lattice through the treatment. Hydrogen reduces Ti4+ to Ti3+ and increases the concentration of charge carriers. The degradation was found to exhibit a strong dependence on time at room temperature. The degraded properties were spontaneously recovered through an aging process, showing a spontaneous recovery unique to TiO2-based ceramic capacitors. It is proposed that hydrogen is metastable in TiO2 and that hydrogen-induced degradation has different stabilities among various oxide-based components and devices.

Comparison of LuYAP, LSO, and BGO as scintillators for high resolution PET detectors

For high resolution positron emission tomographs based on scintillation detectors a fast, dense, and bright scintillator is required. A sample of a new scintillator, Lu/sub 0.8/Y/sub 0.2/AlO/sub 3/:Ce (LuYAP) with a density of 7.7 g/cm/sup 3/ and a scintillation decay time of 20 and 160 ns is compared with LSO and BGO crystals of the same size to estimate the potential of the crystal for high resolution PET detectors. Special attention was paid to use an application specific measurement setup with respect to high resolution PET. The light yield of the crystals using polished crystals as well as PTFE tape and BaSO/sub 4/ as reflector material and the temperature dependence of the light yield are measured using crystals of 2 /spl times/ 2 /spl times/ 10 mm/sup 3/. At room temperature, the light yields of BGO : LuYAP : LSO are 1 : 1.5 : 4.5 with energy resolutions of 27.4% (BGO), 20% (LuYAP) and 15% (LSO), respectively. LuYAP shows an increase in light yield with increasing temperature whereas both BGO and LSO show a decrease.

Enhancement of band-edge photoluminescence of bulk ZnO single crystals coated with alkali halide

We have studied coating effects of bulk ZnO crystal by means of photoluminescence (PL) measurements. Bulk crystal surface of ZnO is once covered by the melt of alkali halide at high temperature and is coated with thin alkali halide crystal. At low temperature, band-edge PL intensity of the ZnO crystal coated with KCl or KI is about 300 times larger than that of a polished crystal. In the no-coating ZnO crystal treated by the same thermal conditions, the enhancement of the PL intensity is only about 20 times larger. The enhancement of band-edge PL also occurs considerably at room temperature. The experimental results including the PL spectra in the visible light region, suggesting the qualitative improvement of ZnO surface by the coating, are discussed in terms of the incorporation of halogen ions supplied from alkali halide into oxygen vacancies.

A room temperature indium tin oxide/quartz crystal microbalance gas sensor for nitric oxide

Nitric oxide (NO) gas sensor using a combination of indium tin oxide (ITO) and quartz crystal microbalance is fabricated. The sensor is made from the polished AT-cut quartz crystal on which the ITO thin film is deposited using radio frequency magnetron sputtering. The sensor is sensitive to NO gas at room temperature and avoids the problems related to elevated-temperature sensors. It is shown that the sensor has a distinct negative frequency shift of 110 Hz within 600 s when it is exposed to 60 ppm of NO. The frequency-changing rate is also found to increase with NO concentration. The chemical status of elements on the ITO surface before and after NO adsorption is investigated by X-ray photoelectron spectroscopy (XPS). Possible gas sensing mechanisms are discussed.